In general, infrared (IR) radiation may have a range of wavelengths of 0.8-20 μm or larger though the most common wavelengths uses for thermal imagery are 3-5 um (commonly known as the mid-wave IR band or MWIR) and 8-12 μm (commonly known as the long-wave IR band or LWIR).
The human eye cannot detect infrared light. But infrared energy can be detected electronically. Sophisticated electronic instruments exist which can scan a scene and convert the infrared light to an electrical signal which can be displayed on a video monitor, analyzed by a computer, or recorded on film. Electrically, the output of these instruments is very similar to the output of a conventional video camera.
IR imaging systems are designed to satisfy different performance parameters, depending on their intended use. Military applications, such as missile guidance, require the highest level of accuracy and reliability.
Due to their complexity, IR imaging systems are expensive, sensitive, high-maintenance devices. To assure proper operation of these systems and to achieve their full performance requires frequent test and calibration. Engineers, who design IR imaging systems, test them during the design and development stage to evaluate performance parameters and to refine designs to optimize performance. Manufacturers of IR imaging systems need to compare actual performance to specifications, and need to calibrate the systems prior to delivery. End users must test their systems regularly to verify proper operation, and must recalibrate them periodically while they are in the working environment.
Some of the important performance characteristics of an IR imaging system are spatial resolution (ability to resolve fine detail), thermal resolution (ability to resolve small temperature differences), speed (ability to respond to a rapidly changing scene without blurring), and dynamic range (how large a temperature span it can view without saturating). Standard tests have been developed to quantify these characteristics.
IR Test Equipment
Setup, test, and calibration of IR imaging systems requires the use of specialized test equipment. This test equipment is designed to create an infrared scene of precisely known characteristics, to project this scene to the input of the IR imaging system being tested, and to evaluate the quality of the output of the IR imaging system.
Infrared Scene Projector (IRSP)
Over the past three decades, infrared scene projection has evolved into a critical laboratory tool for evaluation of high-performance infrared imagers and their embedded algorithms. This technology projects accurate, realistic and dynamic IR scenes into the entrance aperture of the sensor being tested. It is used to simulate the operating environment of various systems, including imaging infrared missile seekers, search and track systems, and thermal imagers. Using it for hardware-in-the-loop simulation has reduced the scope and cost of flight/field testing, while enabling a new level of sensor optimization. Hardware-in-the-loop simulation enables the generation of synthetic IR imagery for laboratory evaluation of high-performance electro-optical systems.
An Infrared Scene Projector (IRSP) may be used to test a wide variety of sensors used by the US military and major defense contractors. Generally, an IRSP comprises a large number of thermal (IR) emitters, arranged in an array of pixel elements, such as 1024×1024 pixel elements.
The IRSP may use a single chip IR emitter array to produce actual thermal imagery. The emitter array utilizes a large number of pixels to generate the image (similar to how a digital camera uses a large number of pixels to capture an image). Each pixel emits thermal energy that is ultimately captured by the sensor under test. There are many types of emitters such as resistive bridges, Light Emitting Diodes (LEDs), lasers, deformable membranes, micro mirror arrays, etc. Of these emitter types, resistive bridge arrays and micro mirrors are the most widely used. Resistive bridges may offer the best performance in terms of temperature range, speed (frame rate and thermal transition time) and thermal resolution.
LEDs as Pixel Elements in an IRSP
A light-emitting diode (LED) is a two-lead semiconductor light source that resembles a basic pn-junction diode, except that an LED also emits light. When an LED's anode lead has a voltage that is more positive than its cathode lead by at least the LED's forward voltage drop, current flows. Electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. The earliest LEDs emitted low-intensity infrared light. Infrared LEDs are still frequently used as transmitting elements in remote-control circuits, such as those in remote controls for a wide variety of consumer electronics. The current-voltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage (see Shockley diode equation). This means that a small change in voltage can cause a large change in current. If the applied voltage exceeds the LED's forward voltage drop by a small amount, the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is to use constant-current power supplies to keep the current below the LED's maximum current rating. And the typical solution to dimming (reducing) the output of an LED is to use pulse width modulation (PWM).
Light emitting diodes (LEDs) are currently in development as sources for infrared scene projection systems (IRSPs). LEDs offer some advantages over other thermal emitters (such as resistive elements), including potentially higher apparent temperatures and tunable output wavelength. However, implementing a high (extended) dynamic range IRSP, covering multiple orders of magnitude of radiance can be difficult to implement with an LED-based thermal emitter using a standard LED driver circuit (or drive circuit, or simply “driver”). (For purposes of the discussions set forth herein, LEDs may be considered to be a type of “thermal emitter”.)
High-Temperature, High-Dynamic Range IRSP
Attention is directed to Design Considerations for a High-Temperature, High-Dynamic Range IRSP, Joe LaVeigne, Breck Sieglinger, Proc. SPIE 8356, Technologies for Synthetic Environments: Hardware-in-the-Loop XVII, 83560G (May 1, 2012); doi:10.1117/12.922984, incorporated by reference herein. As disclosed therein,                Achieving very high apparent temperatures is a persistent goal in infrared scene projector (IRSP) design. Several programs are currently under way to develop technologies for producing high apparent temperatures. Producing a useful system capable of reproducing high fidelity scenes across a large range of apparent temperatures requires more than just a high temperature source. The entire scene projection system must support the extended dynamic range of the desired scenarios. Supporting this extended range places requirements on the rest of the system. System resolution and non-uniformity correction (NUC) are two areas of concern in the development of a high dynamic range IRSP.        Among other qualities, high radiance or apparent temperature is desired for many IR scene projector applications. Current state of the art resistive arrays can achieve apparent temperatures up to 700° K in the 3-5 um band. New technologies are currently under development with the goal achieve temperatures in excess of 2000° K using high temperature materials for resistive arrays or other sources using narrow band emission. While there has been considerable effort applied to the development of these technologies with the basic goal of being capable of producing very high apparent temperatures, there has been less of a focus on the system level aspects of a high dynamic range projector. Although producing higher temperatures is a worthy goal, a practical IRSP system must also produce accurate radiance with high fidelity at low apparent temperatures as well.        System level resolution and non-uniformity correction (NUC) are two areas of concern. Resolution becomes more of a challenge as the maximum apparent temperature of the system increases. This is primarily due to the nonlinear relationship between apparent temperature and radiance. A system with a maximum apparent temperature of 2000° K has a MWIR (mid-wave IR band) output radiance 40 times higher than one with a 700° K maximum apparent temperature. The 40 times larger radiance at 2000° K means a system that can achieve that radiance will require a higher fidelity by that same factor in order to simulate low temperature objects with the same absolute resolution. The resolution of the system is set by that component or algorithm which has the lowest resolution and may be dependent on the radiance being commanded.        The apparent temperature of an object is defined as the temperature of a blackbody that produces the equivalent integrated radiance over the band of interest. The Planck Function shown in Equation 1 describes the radiance of a blackbody.        
                              L          ⁡                      (                          λ              ,              T                        )                          =                                            2              ⁢                              hc                2                                                                    λ                5                            ⁡                              (                                                      θ                                                                  hc                        /                        λ                                            ⁢                                                                                          ⁢                      kT                                                        -                  1                                )                                              ⁢          d          ⁢                                          ⁢          λ                                    [        1        ]                            Consider a system that is linear in radiance with the requirement to have a minimum resolution of 0.1° K at an apparent temperature of 300° K. Such a system would be capable of producing a reasonable simulation of a typical ambient environment, though it would be far from the ˜15 mK (milliKelvins) Noise Equivalent Temperature Difference (NETD) of a typical mid-wave infrared (MWIR) imager. A 0.1° K step at 300K is equivalent to a radiance of 6.8×10−7 W/(cm2sr) in the 3-5 MWIR band. If that same projection system were to be capable of a maximum apparent temperature of 2000° K, the system would require approximately 8 million steps, or nearly 23 bits of resolution. Scene projectors are also used to depict scenes with a space background. To achieve this, the projectors are typically operated inside a cryogenic chamber at temperatures near 100° K (−173° C.) or lower. For these low background situations the resolution needed for practical simulation is comparable to that of the camera, approaching 25 bits, or 40 million steps.        Current scene projectors are designed as 16 bit systems and typically operate linear in radiance. Without changes, these systems would not be acceptable for a high apparent temperature array. For example, a 16 bit linear system with a 2000° K maximum MWIR apparent temperature would have a step size at 300° K of over 20° K, making the system impractical for the simulation of typical terrestrial temperature scenes.        For a system with a 2000° K MWIR maximum apparent temperature, the native resolution will become coarser assuming a similar digital to analog converter (DAC) with an effective resolution of 14 to 15 bits is used. The predicted resolution for such a system is shown in FIG. 3. Though the step size is larger than existing arrays, it does not exceed 0.1° K until nearly 500° K. Based on these predictions, a 14 bit native resolution for a resistive array would be adequate depicting low radiance scenes. FIG. 3 also contains a plot of the resolution of the same system after a 16 bit linearization has been applied. In this case, the step size near ambient temperatures increases to 10° K or more. Such a coarse resolution is not acceptable for low radiance scenes. In order to operate with a response that is linear in radiance, a different representation must be used in the system. This will lead to new firmware and potentially new hardware being developed to support the new representation. A 24 bit fixed point number would be just acceptable for a MWIR projector with a 2000° K maximum temperature. A floating point representation would also be acceptable. Given the flexibility of the floating point representation for future growth, it is the recommended format for the next generation for control electronics.        The resolution issues described above apply to any emitter array. Consider a light emitting diode (LED) based array. In that case the native radiance versus drive function is much closer to being linear than the resistive arrays. If the native bit depth of a system based on a LED emitter array is 16 bits, then that will set the limit on the system resolution. For high temperature LED arrays, a higher resolution circuit at low radiance levels will be required in order to simulate low radiance scenes. The issues with digital micro-mirror devices (DMDs) are related. For a DMD to produce adequate resolution it must be capable of flipping back and forth very rapidly. For a 23 bit system operating at 400 Hz, in order to display a single bit of radiance, the minors would have to switch at over 3 GHz. This is far beyond the maximum mirror frequency of nearly 100 MHz for DMD devices currently in use.        
Another solution would be to use an analog input, but this option also has complications. (An analog solution may have infinite resolution, but is limited by noise.) Assuming the read-in-integrated-circuit (RIIC) has a 0-5V input range, the smallest step size would be 5V/(8×106) or less than 1 μV. Typical high speed DACs (digital-to-analog converters) have noise levels on the order of 10's of μV, making an analog system comparable in complexity to a digital system. Although the range of radiance is very large, the absolute resolution required is not the same over the entire range of the projector. The relative radiance resolution required is on the order of 0.5% of the projected value (radiance emitted by a pixel). In this case, adequate resolution could be achieved with a 16-bit system if a non-linear radiance versus drive were used.